The objective of this research is to estimate birth rates by age and duration since first marriage in Korea (based on the 1975 Census) and the United States (based on the 1970 Census) and to use these rates to analyze fertility differentials and trends. The estimates are derived by means of a new extension of the own-children method of fertility estimation. Rates are estimated for each of the 15 calendar years previous to census enumeration, as well as for groups of calendar years. The detailed age-duration-specific birth rates are aggregated to age-specific ever-marital birth rates and to duration-specific birth rates. The duration-specific rates are aggregated to an ever-marital total fertility rate (EMTFR), which provides a convenient and concise synthetic measure of completed family size for ever-married women. Estimates are produced not only at the national level but also for geographic subdivisions and by selected socioeconomic characteristics asked in the census, most notably rural-urban residence, education, and occupation. The own-children estimates of age-duration-specific fertility by geographic subdivision and socioeconomic characteristics are used in combination with earlier own-children and other estimates of age-specific fertility and marital fertility to analyze fertility differentials and trends. Results of the study should prove useful to health planners for evaluating and improving the effectiveness of family planning programs in different geographic areas and among different socioeconomic groups.